MySQL group by對單字分組序和多字段分組的方法講解
我這里創(chuàng)建了一個 goods 表，先看下里面的數(shù)據(jù)：
mysql> select * from goods; +----+------+------+------------+-------------+------------+ | id | s_id | b_id | goods_name | goods_price | goods_desc | +----+------+------+------------+-------------+------------+ | 1 | 1 | 5 | book | 22.35 | book | | 2 | 2 | 5 | ball | 32.25 | ball | | 3 | 3 | 5 | NULL | 3.23 | NULL | | 4 | 3 | 5 | macbook | 3.23 | book | | 5 | 3 | 5 | listbook | 2.30 | book | | 6 | 1 | 1 | nicebook | 9999.00 | nicebook | | 7 | 2 | 3 | googlebook | 25.30 | book | +----+------+------+------------+-------------+------------+
1、根據(jù)s_id分組
mysql> select *,group_concat(goods_name) goods_names,group_concat(goods_desc) goods_descs,group_concat(id) ids,group_concat(goods_price) goods_prices from goods group by s_id; +----+------+------+------------+-------------+------------+------------------+---------------+-------+----------------+ | id | s_id | b_id | goods_name | goods_price | goods_desc | goods_names | goods_descs | ids | goods_prices | +----+------+------+------------+-------------+------------+------------------+---------------+-------+----------------+ | 1 | 1 | 5 | book | 22.35 | book | book,nicebook | book,nicebook | 1,6 | 22.35,9999.00 | | 2 | 2 | 5 | ball | 32.25 | ball | ball,googlebook | ball,book | 2,7 | 32.25,25.30 | | 3 | 3 | 5 | NULL | 3.23 | NULL | macbook,listbook | book,book | 3,4,5 | 3.23,3.23,2.30 | +----+------+------+------------+-------------+------------+------------------+---------------+-------+----------------+
這里使用了 group_concat()函數(shù)，主要目的是為了顯示分組的詳細(xì)信息
上面的根據(jù)單個字段分組很簡單，把相同s_id的記錄都?xì)w組了
2、根據(jù)s_id,goods_desc字段分組
分析：這里查詢分組時，會先根據(jù)s_id分組，然后對每個組里面的數(shù)據(jù)再根據(jù)goods_desc進(jìn)行分組
mysql> select *,group_concat(goods_name) goods_names,group_concat(goods_desc) goods_descs,group_concat(id) ids,group_concat(goods_price) goods_prices from goods group by s_id,goods_desc; +----+------+------+------------+-------------+------------+------------------+-------------+------+--------------+ | id | s_id | b_id | goods_name | goods_price | goods_desc | goods_names | goods_descs | ids | goods_prices | +----+------+------+------------+-------------+------------+------------------+-------------+------+--------------+ | 1 | 1 | 5 | book | 22.35 | book | book | book | 1 | 22.35 | | 6 | 1 | 1 | nicebook | 9999.00 | nicebook | nicebook | nicebook | 6 | 9999.00 | | 2 | 2 | 5 | ball | 32.25 | ball | ball | ball | 2 | 32.25 | | 7 | 2 | 3 | googlebook | 25.30 | book | googlebook | book | 7 | 25.30 | | 3 | 3 | 5 | NULL | 3.23 | NULL | NULL | NULL | 3 | 3.23 | | 4 | 3 | 5 | macbook | 3.23 | book | macbook,listbook | book,book | 4,5 | 3.23,2.30 | +----+------+------+------------+-------------+------------+------------------+-------------+------+--------------+
這里的goods_descs 和 上面的一比較就明白了
接下來還可以再根據(jù) goods_price 分組
mysql> select *,group_concat(goods_name) goods_names,group_concat(goods_desc) goods_descs,group_concat(id) ids,group_concat(goods_price) goods_prices from goods group by s_id,goods_desc,goods_price; +----+------+------+------------+-------------+------------+-------------+-------------+------+--------------+ | id | s_id | b_id | goods_name | goods_price | goods_desc | goods_names | goods_descs | ids | goods_prices | +----+------+------+------------+-------------+------------+-------------+-------------+------+--------------+ | 1 | 1 | 5 | book | 22.35 | book | book | book | 1 | 22.35 | | 6 | 1 | 1 | nicebook | 9999.00 | nicebook | nicebook | nicebook | 6 | 9999.00 | | 2 | 2 | 5 | ball | 32.25 | ball | ball | ball | 2 | 32.25 | | 7 | 2 | 3 | googlebook | 25.30 | book | googlebook | book | 7 | 25.30 | | 3 | 3 | 5 | NULL | 3.23 | NULL | NULL | NULL | 3 | 3.23 | | 5 | 3 | 5 | listbook | 2.30 | book | listbook | book | 5 | 2.30 | | 4 | 3 | 5 | macbook | 3.23 | book | macbook | book | 4 | 3.23 | +----+------+------+------------+-------------+------------+-------------+-------------+------+--------------+
這里主要是進(jìn)行多個字段分組的時候，只需掌握分組順序后面的字段是根據(jù)前面字段分組后的內(nèi)容再進(jìn)行的分組即可。
在平時的開發(fā)任務(wù)中我們經(jīng)常會用到MYSQL的GROUP BY分組， 用來獲取數(shù)據(jù)表中以分組字段為依據(jù)的統(tǒng)計數(shù)據(jù)。比如有一個學(xué)生選課表，表結(jié)構(gòu)如下：
Table: Subject_Selection
Subject Semester Attendee --------------------------------- ITB001 1 John ITB001 1 Bob ITB001 1 Mickey ITB001 2 Jenny ITB001 2 James MKB114 1 John MKB114 1 Erica
我們想統(tǒng)計每門課程有多少個學(xué)生報名，應(yīng)用如下SQL：
SELECT Subject, Count(*) FROM Subject_Selection GROUP BY Subject
得到如下結(jié)果：
Subject Count
------------------------------
ITB001 5
MKB114 2
因?yàn)楸砝镉涗浟擞?個學(xué)生選擇ITB001，2個學(xué)生選擇了MKB114。
產(chǎn)生這個結(jié)果的原因是：
GROUP BY X意思是將所有具有相同X字段值的記錄放到一個分組里。
那么GROUP BY X, Y呢？
GROUP BY X, Y意思是將所有具有相同X字段值和Y字段值的記錄放到一個分組里。
總結(jié)
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